Method and apparatus for reducing noise in a variable reluctance motor

ABSTRACT

A control circuit for a motor having at least two windings is used to optimize noise reduction by controlling the deenergization of each winding in a two-stage decay. The duration of the two-stage decay and particularly the first decaying current portion is controlled by a controller operatively coupled to a switch device. The duration of the first decaying current portion is varied between the various phases of the motor to provide optimum noise reduction.

This application is a continuation of U.S. patent application Ser. No.08/690,172 filed Jul. 26, 1996, now U.S. Pat. No. 5,742,139.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to a system for controlling a switchedreluctance (SR) motor, and more particularly, to a method for providingnoise reduction for an SR electric motor by controlling the currentprofile in the individual windings.

2. Discussion of the Related Art

Switched Reluctance Machines (SRMs) have been the subject of increasedinvestigation due to their many advantages, which makes them suitablefor use in a wide variety of applications. An SR machine operates on thebasis of varying reluctance in its several magnetic circuits. Inparticular, such machines are generally doubly salient motors--that is,they have teeth or poles on both the stator and the rotor. The statorpoles have windings which form machine phases of the motor. In a commonconfiguration, stator windings on diametrically opposite poles areconnected in series to form one machine phase.

When a machine phase is energized, the closest rotor pole pair isattracted towards the stator pole pair having the energized statorwinding, thus minimizing the reluctance of the magnetic path. Byenergizing consecutive stator windings (i.e., machine phases) insuccession, in a cyclical fashion, it is possible to develop torque, andthus rotation of the rotor in either a clockwise, or counter-clockwisedirection. The inductance of a stator winding associated with the statorpole pair varies as a function of rotor position. Specifically, theinductance varies from a lower level, when a rotor pole is unalignedwith a corresponding stator pole, to an upper or maximum level when therotor pole and stator pole are in alignment. Thus, when a rotor polerotates and sweeps past a stator pole, the inductance of the statorwinding varies through lower-upper-lower inductance levels. Thisinductance-versus-rotor position characteristic is particularly relevantfor controlled operation of the motor. Specifically, current flowingthrough the stator winding must be switched on (e.g., via powerelectronics) prior to, and maintained during the rising inductanceperiod in order to develop a positive torque. Since positive phasecurrent in the decreasing inductance interval produces a negative orbreaking torque, the phase current must be switched off (e.g., bydeenergizing the power electronics) before this interval occurs in orderto avoid generating negative torque. Accordingly, rotor position sensingis an integral part of a closed-loop switched reluctance motor drivesystem so as to appropriately control torque generation.

The simple construction, low cost and fault tolerance of the powerelectronic drive circuitry of SRMs has made them desirable forapplications in industrial and automotive applications. Due to thenature of the motor geometry and drive circuitry, SR motors may exhibitan undesirable level of audible noise compared to that of other types ofmotors. Reducing audible noise has thus been investigated in the priorart.

One source of noise has been traced to the deforming of the rotor andstator. In SR motors, the stator poles form a generally cylindricalchamber in which the rotor rotates the rotor poles form a generallycylindrical cross section. The rotor tends to align with the stator asthe magnetic forces cause the rotor to be attracted to the stator. Themagnetic forces cause the desired angular movement of the rotor relativeto the stator. These forces, however, also tend to "ovalize" the rotorand stator. Ovalization occurs due to the magnetic forces deforming therotor and stator in the direction of the forces. The audible noise isgenerated as the current in the winding is abruptly switched off due tothe rotor and stator quickly returning to their undeformed shape.

One solution to reduce audible noise teaches identically controlling thedeenergizing of each winding in two stages to gradually reduce thecurrent so that a less abrupt change takes place. In this solution, eachwinding is energized using a higher current flow than that normallyused. The current flow to the winding is then gradually reduced. Whenthe winding reaches a commutation point; that is, when the motor isdisconnected from the current source, the winding is allowed tonaturally de-energize. Since the current is gradually reduced, lessnoise is generated by the motor. This technique, however, has been foundto reduce noise only to a certain extent. Further noise reductions,however, are still desirable. Consequently, a need exists to provide animproved reduced noise switched reluctance machine to alleviate one ormore of the problems set forth above.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved controlstrategy for operating a motor, and has particular application tofurther reduce the noise in an SR motor beyond that provided byconventional approaches. The control strategy compensates for variationsin the resonant frequencies throughout the motor geometry. The presentinvention recognizes that the current profiles for each motor phase mustbe considered individually to further reduce noise whereas conventionalstrategies treat each motor phase the same, that is, by providing thesame current profile to each phase.

A preferred embodiment of the present invention provides a switch usedto selectively couple current signals to the windings of the motors. Acontrol means controls the switches to form an optimum current profilefor each of the windings so that noise reduction in each of the motorphases is maximized. The optimum current profile for each winding isexperimentally determined. Each profile varies between the variouswindings. These values are then stored in the memory of the controller.

In one aspect of the invention the current profile has an increasingcurrent portion and a first and second decaying current portion. Theduration of the first decreasing current portion is varied between thevarious phases of the motor.

In a preferred method for operating the motor, the controller retrievesthe desired current profile for a particular winding from memory andenergizes the winding according to the predetermined profile. Thecurrent profile information is used to de-energize the windingpreferably in a two stage decay when the current in the winding isturned off (at the commutation point). The first stage of the two stagedecay varies between the different windings. The duration of the firstdecaying current portion may be lengthened or shortened between windingsto minimize audible noise. Each of the windings are likely to havedifferent durations for the first decaying current portion.

The advantage of the present invention is that significant noisereductions have been achieved relative to conventional approaches sincethe current profile in each winding is tailored to reduce vibration.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will becomeapparent from the following detailed description which should be read inconjunction with the drawings in which,

FIG. 1 is an exploded, perspective view of a portion of a switchedreluctance electric motor suitable for use in connection with thepreferred embodiment of the present invention;

FIG. 2 is a diagrammatic, exaggerated, cross-sectional view of aswitched reluctance electric motor illustrating the relative positionsof a stator and rotor portions thereof;

FIG. 3 is a schematic/block diagram of a two switch per phase motorcontrol circuit;

FIGS. 4a through 4c are current profiles of three windings of a motorillustrating varying first stage decay;

FIG. 5 is a schematic/block diagram of a one switch per phase motorcontrol circuit;

FIG. 6 is a timing diagram showing the control signal for operating theswitch of FIG. 5;

FIG. 7 is a flow chart of a method for determining the optimum currentprofile; and

FIG. 8 is a flow chart showing the operation of the control circuitaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, like reference numerals are used toidentify identical components in the various views. Although theinvention will be described and illustrated in the context of a switchedreluctance (SR) electric motor 10, it will be appreciated that thisinvention may be used in conjunction with other well-known electricmotor structures.

FIG. 1 shows the major mechanical components of a switched reluctanceelectric motor 10, which includes a stator assembly 12 and a rotorassembly 14. Stator assembly 12, in a preferred embodiment, comprises aplurality of laminations 16. Laminations 16 are formed using amagnetically permeable material, such as iron.

Stator 12 is generally hollow and cylindrical in shape. A plurality ofradially, inwardly extending poles 18 are formed on stator 12 (vialaminations 16) and extend throughout the length thereof. Poles 18 arepreferably provided in diametrically opposed pairs. The illustratedembodiment shows six poles 18. It should be appreciated, however, that agreater or lesser number of poles 18 may be provided in a particularconfiguration.

Each of the poles 18 may have a generally rectangular shape, when takenin cross-section. The radially innermost surfaces of the poles 18 areslightly curved so as to define an inner diameter representing a bore20. Bore 20 is adapted in size to receive rotor assembly 14.

Rotor assembly 14, when assembled into stator 12 (best shown in FIG. 2)is coaxially supported within stator 12 for relative rotational movementby conventional means. Rotor assembly 14, for example, may be supportedby conventional bearings (not shown) mounted in conventional end bells(not shown) secured to the longitudinal ends of stator 12. Rotorassembly 14 includes a generally cylindrical shaft 22 and rotor 24.Shaft 22 may be solid, although illustrated in FIG. 1 as being hollow.Rotor 24 is secured to shaft 22 for rotation therewith. For example,rotor 24 may be secured to shaft 22 by means of a spline (not shown), orother conventional means well-known in the art. Thus, it should beappreciated that shaft 22, and rotor 24 rotate together as a unit.

Rotor 24 includes the plurality of poles 26 formed on an outer surfacethereof. Each pole 26 extends radially outwardly from the outer surfacethereof and is formed having a generally rectangular shape, when takenin cross-section. Rotor poles 26 extend longitudinally throughout theentire length of the outer surface of rotor 24. The radially outermostsurfaces of rotor poles 26 are curved so as to define an outer diameter,adapted in size to be received within the inner diameter defining bore20. That is, the outer diameter formed by poles 26 is slightly smallerthan the inner diameter defined the radially innermost curved surfacesof stator poles 18. Rotor poles 26 may also be provided in diametricallyopposed pairs. Four (4) rotor poles 26 are provided on the illustratedrotor assembly 14; however, it should be appreciated that a greater orlesser number of rotor poles 26 may be provided. For SR motors, ingeneral, the number of rotor poles 26 differs from the number of statorpoles 18, as is well-known. Rotor 24, including poles 26, may be formedfrom a magnetically permeable material, such as iron.

Referring now to FIG. 2, a diagrammatic view of a cross-section of anassembled motor 10 is illustrated. In particular, as referred to above,poles 18 occur in pairs: i.e., AA', BB', and CC'. Rotor poles 26 alsoappear in pairs. Stator windings 28 (shown only on stator pole pair AA'for clarity) of diametrically opposite poles (e.g., AA') associated withstator 12 are connected in series to form one machine phase. Thus, thewindings 28 on poles AA' are referred to as "machine phase A" of SRmotor 10.

In the illustrated example, SR motor 10 also has a machine phase B, anda machine phase C. Each of these three machine phases may be energizedindividually, which, when done in a controlled manner, provides forrotation of rotor 24. Although a three-phase machine is described andillustrated, a machine having any number of phases greater than one iscontemplated as falling within the spirit and scope of the presentinvention.

Referring now to FIG. 3, the current level within the phases of themotor is controlled by the operation of either one or two switches. FIG.3 illustrates a two switch per phase embodiment. Only one phase 29 ofthe two switch per phase configuration is shown since each of the phaseshave identical configurations.

A controller 30 is used to control the operation of switches 32 and 34.Controller 30 is preferably a microprocessor, but one skilled in the artwould recognize that discrete components may also be used.

Switches 32 and 34 are separately electrically controllable to connectwinding 28 to and from a power supply 36. Switches 32 and 34 arepreferably transistors and most preferably MOSFETS. Switches 32 and 34may also be a variety of well-known switching devices such asthyristors, relays and the like.

Controller 30 controls the operation of switches 32 and 34 through gatedrives 38 and 40. Gate drives 38 and 40 are conventional and well-knownin the art and are used to amplify the control signal from controller 30to provide the proper biasing to control the operation of switches 32and 34.

Power supply 36 is also conventional and well-known in the art. Powersupply 36 is sized to provide current signals to achieve a desired levelof current to drive each phase 29 of motor 10.

Controller 30 controls the output of the motor, e.g., speed and torque,based on an input 42 that provides feedback as to the proper output ofthe motor. Input 42 may be a manually operated external control switch,feedback signals from another controller or the like (not shown).Parameters of input 42 may include phase current magnitude and angularposition of rotor 24.

A memory 44 is used to store the characteristics for a desired currentprofile to correspond to inputs 42. For example, to provide a desiredtorque, current levels and timing for the current may be stored. Inparticular, commutation data, and switch control data for controllingthe conduction of switches 32 and 34 may be provided. Memory 44 is shownas a separate component, however, memory 44 may be an integral part ofcontroller 30.

A pair of diodes 46 and 48 are connected between winding 28 and powersupply 36 (i.e., respective negative and positive busses thereof) toprovide current paths for different operating modes of the windings. Thefunction of diodes will become apparent in the description below.

Referring now to FIGS. 3 and FIGS. 4a through 4c, the operation of thecontrol circuitry of the motor is best understood while referring to thetiming diagram. The portion of the current profile between time T₀ andtime T_(C) is the increasing current portion. During the increasingcurrent portion, winding 28 is connected to power supply 36 to energizewinding 28 to thereby provide the proper output torque.

The period between time T_(C) and T_(D) is the first decaying currentportion 33 of the current profile. During the first decaying currentportion 33, either switch 32 or 34 is opened. Preferably, top switch 32is controlled to a non-conductive state by controller 30 through gatedrive 38. Time T_(C) is referred to as the commutation point. At thecommutation point T_(C), current is no longer supplied by supply 36 tothe motor windings. When the selected switch is opened, the motor enterswhat is commonly referred to as a freewheeling state in which thecurrent in the winding circulates between winding 28 and one of diodes46 or 48. If switch 32 is open, diode 48 and winding 28 form thefreewheeling circuit. If switch 34 is open, diode 46 and winding 28 formthe freewheeling circuit. It should be appreciated that in thefreewheeling state, energy no longer is being supplied to or removedfrom winding 28 through power supply 36. The current in the winding 28will gradually be reduced through electrical losses in the winding andassociated circuitry. Because the rate of change of the first stage ofdecay is dependent on the electrical circuitry connected thereto, oneskilled in the art would recognize that other components may be switchedinto the freewheeling circuit to alter the decay characteristics.

The duration of first decaying current portion is controlled by theoperation of controller 30 so that a desired profile is obtained. Theduration of the first decaying current portion to achieve maximum noisereduction is stored in memory 44. Controller 30 reads that duration atthe appropriate time.

The period between time T_(D) and time T_(E) is a second decayingcurrent portion 35 of the current profile. During the second decayingcurrent portion 35, both switches 32 and 34 are open. The remainingcurrent from winding 28 is forced in a reverse direction through diodes46 and 48. The reverse direction causes a rapid decay of the remainingcurrent in winding 28. Once again the rate of decay for the second decayperiod is determined by the characteristics of the electrical componentsconnected into the circuit at that time. Of course, by switching inadditional electrical components, the rate of change may be altered asdesired.

Referring now to FIGS. 4a through 4c, the first decaying current portion33, preferably varies between the phases of the motor. FIGS. 4a through4c represent the three windings of a three phase motor. In oneembodiment, the duration of the first decaying current portion 33 ofFIG. 4a is 150 microseconds. The first decaying current portion 33 ofFIG. 4b is controlled to be 100 microseconds. In FIG. 4c, the firstdecaying current portion 33 is reduced to 50 microseconds. In theparticular configuration of the motor, these settings wereexperimentally determined to optimally reduce noise. Of course, if adifferent geometry motor was used, the setting for the first decayingcurrent portion 33 would be altered to optimize audible noise.

Referring now to FIG. 5, like reference numerals are used to indicateidentical components as shown in FIG. 3. Once again only a single phase49 is shown. FIG. 5 shows a one switch per phase configuration that iscommon in motor drives. The operation is varied over conventionalcircuits to achieve the desired current profile such as that in FIG. 4.In this configuration, only one diode 54 is used. Such a configurationis used typically in a motor having an even number of machine phases. Aswitch 50 is used to control the current level in winding 52. Adjacentphases alternate the position of diode 54 and switch 50 as would beevident to one skilled in the art. Capacitors 56 and 58 are connectedbetween power supply 36 and winding 52 and allow regeneration of thecurrent. The current level in winding 58 is controlled by controller 30through a gate drive 60. Winding 52 is powered when switch 50 is closed.Winding 52 is unpowered when switch 50 is open. To control thedeenergization of winding 52 to achieve a two-stage decay such as thatshown in FIG. 4, switch 50 must be pulsed since a configuration usingonly one switch allows only a connection to or disconnection from powersupply, i.e., no freewheeling state is obtainable.

Referring now to FIGS. 4, 5 and 6, the operation of the one switch perphase circuit is described in conjunction with the desired currentprofiles. The same current profiles as that of FIG. 4 are desired.Between time T₀ and time T_(C), controller 30 closes switch 50 toconnect winding 52 to power supply 36 to achieve an increasing currentprofile. A pulse train 61 such as that shown in FIG. 6 may be used toconform the current profile to a desired level. Pulse train 61 has aduty cycle that varies from a high duty cycle at time T₀ to a lower dutycycle approaching the commutation time T_(C). One skilled in the artwould also recognize various other methods of obtaining the desiredoutput such as changing the frequency or a combination of pulse widthmodulating and varying the frequency.

During the first decaying current period 33, the current flow isessentially discontinued to achieve the desired profile. To achieve aslower rate of decay switch 50 may be pulsed a few times. The minimalpulsing has a net effect of controllably reducing the current in winding52. If a slower rate of decay is desired between time period T_(C) andtime T_(D), fewer or shorter pulses may be used. If a faster rate ofdecay is desired, more pulses or longer pulses may be used.

Controller 30 is used to provide pulse train 61 to gate drive 60 toenergize and deenergize winding 52. Memory 44 stores the number andduration of the desired pulse train for each portion of the currentprofile so that the duration of the period between T_(C) and T_(D) isvaried to optimize noise reduction between the various motor phases.

Referring now to FIG. 7, the method for determining the optimum currentprofile for each stage is shown. In step 70, a first winding isenergized. In step 72, the winding is deenergized according to atwo-step decay process. The audible noise is then measured in step 74.The noise can be measured by mounting velocity or acceleration sensorsor by using a microphone to analyze the audible noise produced by themotor during operation in a conventional manner. In step 76, the noiseis measured to determine whether a maximum reduction in noise has beenachieved. In step 78, if the maximum noise reduction has not beenachieved, the duration of the first stage of decay is adjusted and theprocess returned to step 70. Once the maximum noise reduction has beenobtained in the first winding, the current profile (particularly theduration of the first stage of decay) is stored in the memory of thecontroller in step 80. Typically, the maximum noise reduction for thefirst phase is obtained using a duration for the first decaying currentportion of about half a period of the resonant frequency of the motor.

The optimization process must be completed for each winding. Step 82determines whether all the windings have been optimized. If theoptimization has not been completed for each winding, step 70 iscompleted each time through the process. After the first time throughthe process, step 70 also energizes the previously tested windings. Forexample, the second time through the loop, both the first and secondwindings are energized so that the current profile of the second windingcan be optimized. For the third winding, the first and second windingsare energized according to their optimized current profiles.

Once the last winding has been optimized, step 86 ends the process. Theoptimized current profiles are all stored in memory 44 of controller 30for use during operation of the motor.

Referring now to FIG. 8, the preferred method of operating a motorhaving noise reduction optimization is described. In step 88, beforestarting up the motor, controller 30 retrieves the current profileinformation for a winding from memory 44. In step 90, the winding isenergized according to the retrieved current profile. In step 92, thewindings are deenergized for a first decaying current portion for afirst duration. Each winding is likely to have different durations forthe first decaying current portion. In step 94, the current iscompletely dissipated from the winding during a second decaying currentportion. The second decaying current portion occurs at a faster ratethan the first decaying current portion. If the motor is still on instep 96, step 88 is repeated for each of the windings.

While the best mode for carrying out the present invention has beendescribed in detail, those familiar with the art to which this inventionrelates will recognize various alternative designs and embodiments forpracticing the invention as defined by the following claims:

What is claimed is:
 1. An apparatus, comprising:means for energizing afirst winding and a second winding of a motor; means for controlling afirst current in said first winding according to a first current profileand for controlling a second current in said second winding according toa second current profile; and, a memory for storing a first set of datacorresponding to said first current profile and a second set of datacorresponding to said second current profile wherein said first currentprofile is different than said second current profile.
 2. The apparatusof claim 1 wherein each of said first and second current profilesincludes a decreasing current portion and said decreasing currentportion of said first current profile is different than said decreasingcurrent portion of said second current profile.
 3. The apparatus ofclaim 2 wherein said decreasing current portion of said first currentprofile has a first duration and said decreasing current portion of saidsecond current profile has a second duration, said first durationdifferent than said second duration.
 4. The apparatus of claim 1 whereineach of said first and second current profiles includes an increasingcurrent portion and a decreasing current portion having a first stageand a second stage.
 5. The apparatus of claim 4 wherein said first stageof said first current profile is different than said first stage of saidsecond current profile.
 6. The apparatus of claim 5 wherein said firststage of said first current profile has a first duration and said firststage of said second current profile has a second duration, said firstduration different than said second duration.
 7. The apparatus of claim1, further including means for providing a feedback input signal whereinsaid controlling means is responsive to said feedback input signal. 8.An apparatus, comprising:means for energizing a first winding and asecond winding of a motor wherein said energizing means includesfirst,second, third, and fourth switches, said first and second switchesconnected to said first winding and said third and fourth switchesconnected to said second winding; and means for controlling a firstcurrent in said first winding according to a first current profile andfor controlling a second current in said second winding according to asecond current profile wherein said first current profile is differentthan said second current profile, said controlling means includingmeansfor opening said first switch to form a first decreasing portion of saidfirst current profile; means for opening said second switch a firstpredetermined time after opening said first switch to form a seconddecreasing portion of said first current profile; means for opening saidthird switch to form a first decreasing portion of said second currentprofile; and, means for opening said fourth switch a secondpredetermined time after opening said third switch to form a seconddecreasing portion of said second current profile, said firstpredetermined time different than said second predetermined time.
 9. Anapparatus, comprising:means for energizing a first winding and a secondwinding of a motor, said energizing means includingfirst and secondswitches connected to said first and second windings, respectively; andmeans for controlling a first current in said first winding according toa first current profile and for controlling a second current in saidsecond winding according to a second current profile wherein said firstcurrent profile is different than said second current profile, saidcontrolling means includingmeans for pulsing said first switch for afirst predetermined time to form a first decreasing portion of saidfirst current profile; and, means for pulsing said second switch for asecond predetermined time to form a first decreasing portion of saidsecond current profile, said first predetermined time different thansaid second predetermined time.
 10. A method for controlling a motor,comprising the steps of:energizing a first winding; controlling a firstcurrent level in said first winding according to a first currentprofile; energizing a second winding; controlling a second current levelin said second winding according to a second current profile whereinsaid first current profile is different than said second current profileand wherein said step of controlling a first current level includes thesubstep of decreasing said first current level in a first stage and asecond stage and said step of controlling a second current levelincludes the substep of decreasing said second current level in a thirdstage and a fourth stage, said third stage different than said firststage.
 11. The method of claim 10 wherein said first stage has a firstduration, said third stage has a second duration, and said firstduration is different than said third duration.
 12. The method of claim10 wherein said step of controlling a first current level includes thesubstep ofincreasing said first current level to a first commutationpoint, and said step of controlling a second current level includes thesubstep ofincreasing said second current level to a second commutationpoint.
 13. The method of claim 12 wherein said first stage has a firstduration, said third stage has a second duration, and said first stageis different than said third stage.
 14. The method of claim 10, furtherincluding the step of providing a feedback input signal.
 15. A methodfor controlling a motor, comprising the steps of:energizing a firstwinding; controlling a first current level in said first windingaccording to a first current profile; energizing a second winding;controlling a second current level in said second winding according to asecond current profile wherein said first current profile is differentthan said second current profile providing first, second, third, andfourth switches, said first and second switches connected to said firstwinding, said third and fourth switches connected to said second windingwherein said step of controlling a first current level includes thesubsteps ofopening said first switch to form a first decreasing currentportion of said first current profile; and opening said second switch afirst predetermined time after opening said first switch to form asecond decreasing current portion of said first current profile, andsaid step of controlling a second current level includes the substepsofopening said third switch to form a first decreasing current portionof said second current profile; and opening said fourth switch a secondpredetermined time after opening said third switch to form a seconddecreasing current portion of said second current profile, said firstpredetermined time different than said second predetermined time.
 16. Amethod for controlling a motor, comprising the steps of:energizing afirst winding; controlling a first current level in said first windingaccording to a first current profile; energizing a second winding;controlling a second current level in said second winding according to asecond current profile wherein said first current profile is differentthan said second current profile providing first and second switchesconnected to said first and second windings, respectively wherein saidstep of controlling a first current level includes the substep ofpulsing said first switch for a first predetermined time to form a firstdecreasing current portion of said first current profile and said stepof controlling a second current level includes the substep of pulsingsaid second switch for a second predetermined time to form a firstdecreasing current portion of said second current profile, said firstpredetermined time different than said second predetermined time.
 17. Amethod for controlling a motor, comprising the steps of:retrieving afirst set of data corresponding to a first current profile from amemory; and, retrieving a second set of data corresponding to a secondcurrent profile from said memory energizing a first winding; controllinga first current level in said first winding according to said firstcurrent profile; energizing a second winding; controlling a secondcurrent level in said second winding according to said second currentprofile wherein said first current profile is different than said secondcurrent profile.
 18. An apparatus, comprising:means for energizing afirst winding of a first motor phase of a variable reluctance motor anda second winding of a second motor phase of said variable reluctancemotor; and, means for controlling a first current in said first windingaccording to a first current profile from a memory and for controlling asecond current in said second winding according to a second currentprofile from said memory wherein said first current profile is differentfrom said second current profile.